This project seeks to determine the location and mechanism of neurotransmitter secretion and reception. Rapid freezing and subsequent freeze-fracture of synapses expose fleeting structural changes in the cell membrane accompanying discharge of synaptic vesicles. By these means, the prodromata and aftermath of synaptic vesicle exocytosis have been determined. This approach has been extended to other secretory cells where details surrounding the initiation of secretion can be better understood. New methods have been developed to use rapid freezing to localize calcium in neural tissue in different states of activity in order to define its role in controlling these states. These methods have succeeded in defining a special intracellular membrane system which stores calcium. This work is significant in that it defines the normal structure of synapses and relates normal variations in structure to different functional states. Thus, it becomes possible to distinguish pathological changes in structure. The current program also includes freeze-fracture of developing and degenerating synapses which will aid in understanding of both normal development and development failures in the brain and peripheral nervous system.